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Opamp Design with (80dB-100dB) gain

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Member level 2
May 28, 2005
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I need to design an Opamp which gives a gain of 80-100dB with 1.8V power supply. I need the advise/guide from the experience people in this forum. Which configuration will be fine to design such apamp which can gives that amout of gain.

I have situation here, if I use just 2 stages which is telescopic and another gain stage, i might able to achieve such high gain but then the supply voltage will be a limitation for telescopic opamp.

2nd situation is, i can design such gain with 3 stage opamp, but then the risk is on the stability as multistage opamp is a bit hard to make stable.

I have heard about folded cascode (PMOS/NMOS) input stages, may be i can try this.

But anyhow, hope there are more suggestions from the experince guy here.


you didn't tell the input and output swing and the GBW and Cload requirements for the design. you can probably squeeze out that much gain from 2-stage, if it doesnt hit your GBW spec. Avoid three stages if possible. Folded cascode can also be tried..but you need to know the details i mentioned above...


which process are u using, minimum feature size.

u can easily get the 80db gain or even 100 db in a three stage folded cascode architecture, i would refere to the following paper...make sure to get it.

"a compact power efficient 3 v cmos rail to rail input output operational amplifier for vlsi cell libraries".....althought this paper was ment for 3V.....i have used it for 3.3V and 1.8V with no problems what so ever.....

another piece of advice, don't bother with the gm control subcircuitry if u can have fairly stable supply voltages.....your gain will only change by 4 to 6 go for a bit higher gain than your minimum requirements.

depending on your GBW....compensation caps could be as low as 5pf or as high as 11 or 14pf......if your choose to include gm control circuitry, your caps will get as low as 2pf.

i hope this helps.....get the paper man
Read the paper posted by Opamp in the second post.

u can use gain-boosted folded cascode topology to get the desired gain(80db-100db).if your settling time requirement is not too stringent and if you need rail-to-rail o/p,u can use folded cascode opamp followed by a common-source gain stage.
hope it helps.
dont forget to click helped button if it really helped doesn't cost you any point.

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